Methods for removal of impurities from limestone and calcium oxide during the production of hydrated lime

ABSTRACT

Systems and methods for impurity removal to limestone using modifications to the typical calcination process of turning calcium carbonate into calcium oxide prior to slaking to produce calcium hydroxide. Specifically, substantially increasing the temperature to 1100° C. or higher and increasing soak time can result in reductions in certain undesirable impurities, particularly with regards to lead and lead compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Utility patent applicationSer. No. 16/795,306, filed Feb. 19, 2020, which claims the benefit ofU.S. Provisional Patent Application Ser. No. 62/807,556, filed Feb. 19,2019. The entire disclosure of all the above documents is hereinincorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

This disclosure is related to the field of calcium productsmanufacturing and more particularly to manufacturing calcium oxide andhydrated lime with a reduced amount of impurities, particularly lead.

Description of the Related Art

Calcium carbonate (CaCO₃), more commonly known as lime or limestone, isa naturally occurring sedimentary rock having a number of industriallyuseful applications usually due to it being a source for calcium. Aswith most substances, the practical and industrial uses of thesubstances are related to the chemical composition and structure of thesubstance. This is because the uses of the substances rely on thechemical composition in reactions.

In its naturally occurring form calcium carbonate is generally a chalkywhite-ish solid commonly referred to as limestone but there issubstantial variation in color and brightness across limestone fromdifferent sources. In particular, variations based on the geographicalregion and the geological environment from which it is mined are commonand expected. Similarly, calcium carbonate from mined sources istypically different from that of precipitated or manmade sources. Whilesome limestone is naturally very white, others may have an ashen or graytone or can be colored yellow or pink depending on mineral impuritiespresent in the limestone. Due to the usefulness of calcium carbonate asa filler in paints, pigments, and paper, brighter white calciumcarbonates are usually more commercially desirable and considered higherquality.

In practice, most substances we encounter in our everyday lives are notpure. Even filtered drinking water is not exclusively composed of H₂O,and contains some small amount of unwanted inclusions, as well asdesired inclusions, such as fluoride. In practice, virtually nosubstance is completely “pure,” and even artificially purifiedsubstances, such as precious minerals and metals, contain at least traceamounts of inclusions and impurities. Chemical purity is assessed on acase-by-case basis depending upon the particular substance in question,and how it is intended to be used. Calcium carbonate, particularly inits naturally occurring form of limestone, frequently contains a largenumber of inclusions and impurities, in large or small amounts. Commoninclusions in limestone include magnesium, clay, silica, iron compounds,and sulfates. Some inclusions, in some quantities, are acceptable forcertain uses, but others are problematic. Some of the more problematicmaterials in limestone can be minerals that are naturally darker, asthese can lower the brightness of the limestone, and those that cancause problems for other chemical products made from calcium carbonate.

Two particularly useful chemicals made from naturally occurring calciumcarbonate are calcium oxide (CaO), which is commonly known as quicklimeand is typically used in making steel, cement, and other industrialproducts (as well as in some food applications), and calcium hydroxideor hydrated lime (Ca(OH)₂), which can be used in food additives, waterpurification, and pollution mitigation.

Typically both calcium oxide and calcium hydroxide are made from calciumcarbonate. Specifically, ground calcium carbonate is heated (orcalcined) in a lime kiln to a temperature above 825° C. to formquicklime (calcium oxide) in accordance with the following formula:CaCO₃(s)→CaO(s)+CO₂(g)To form hydrated lime, the quicklime is typically subjected to finegrinding or milling to produce a finer material which can be of varyingsizes in different embodiments. Any grinding or milling is suitable,including, for example, fine grind cage mill, swing hammer mill, screenmill, etc. where the amount of milling produces the desiredparticle-size distribution. Once a certain percentage of all the groundparticles meet a desired size (e.g., 95% or smaller than 100 mesh), theground quicklime meeting the desired size requirements is fed into ahydrator, where the calcium oxide reacts with water (also known asslaking), and is then dried (commonly in an environment relatively freeof CO₂) to form calcium hydroxide in accordance with the followingequation:CaO+H₂O→Ca(OH)₂Finally, the resultant calcium hydroxide is then milled and classifieduntil it meets desired criteria for its end use.

Impurities in the original limestone often are carried into theresultant calcium oxide and calcium hydroxide compositions which areproduced above. Depending on the uses of these materials, certainimpurities can create substantial problems. One such concerning impurityis lead which, if carried into resultant calcium hydroxide, can createproblems in pollution mitigation compounds (since it is a controlledpollutant) and in food applications (as it is highly toxic to humans).However, as a naturally occurring substance, determining the amount oflead in original limestone can be problematic which can also result inconcerns in determining the reduction of lead necessary to be performedto provide safe end products efficiently.

Further, while lead is a readily identifiable problematic inclusion, itis not the only one. For example, US Patent Application Publication2007/0104630 and U.S. Pat. Nos. 2,021,663 and 2,127,626 (the entiredisclosure of all of which are herein incorporated by reference) attemptto remove fluoride (another potentially problematic inclusion) fromcalcium oxide and hydrated lime.

While the above patents and applications provide for methods to reducefluoride, this is only a single impurity that presents concerns.Therefore, there is a need in the art to purify limestone to reduce themass of a number of impurities in a given mass of resultant calciumoxide and/or calcium hydroxide and particularly to remove lead.

SUMMARY OF THE INVENTION

The following summary of the invention is provided to give the reader abasic understanding of some aspects of the invention. This summary isnot intended to identify key or critical elements of the invention or todelineate the scope of the invention. The sole purpose of this sectionis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented in a latersection.

Because of these and other problems in the art, described herein, amongother things, are systems and methods for impurity removal to limestoneusing modifications to the typical calcination process of turningcalcium carbonate into calcium oxide. Traditional calcination of calciumcarbonate has not been shown to remove certain heavier elements from thelimestone compound, but an alternative process may be employed whichdoes produce reduced content of certain heavy elements. Specifically,substantially increasing the temperature along with increases to soaktime can result in reductions in lead and certain other heavier and/orproblematic elements.

In an embodiment, there is described a method of calcining limestonecomprising: providing limestone including calcium carbonate andimpurities, said impurities including lead; and calcining said limestoneto produce a calcium oxide composition, said calcining occurring at atemperature of 1100° C. or higher and having a soak time greater than 1hour; wherein an amount of said lead in said calcium oxide compositionis reduced compared to an amount of said lead in said limestone.

In an embodiment of the method, the calcining occurs at a temperature of1200° C. or greater, 1300° C. or greater, 1400° C. or greater, 1500° C.or greater, or 1600° C. or greater.

In an embodiment of the method the soak time is 2 hours or greater.

In an embodiment of the method, the amount of said lead in said calciumoxide is no more than 75% said amount of said lead in said limestone, nomore than 55% said amount of said lead in said limestone, no more than25% said amount of said lead in said limestone, or undetectable.

There is also described herein, in an embodiment a method of calcininglimestone comprising: providing limestone including calcium carbonateand impurities, said impurities including cadmium, cerium, cesium,cobalt, copper, fluoride, lead, lithium, potassium, rubidium, sodium,thorium, and zinc; and calcining said limestone to produce a calciumoxide composition, said calcining occurring at a temperature of 1100° C.or higher and having a soak time greater than 1 hour; wherein an amountof said impurities in said calcium oxide composition is reduced comparedto an amount of said impurities in said limestone.

In an embodiment of the method, the calcining occurs at a temperature of1200° C. or greater, 1300° C. or greater, 1400° C. or greater, 1500° C.or greater, or 1600° C. or greater.

In an embodiment of the method the soak time is 2 hours or greater.

There is still further described herein, a method of calcining limestonecomprising: providing limestone including calcium carbonate andimpurities, said impurities including fluoride; and calcining saidlimestone to produce a calcium oxide composition, said calciningoccurring at a temperature of 1100° C. or higher and having a soak timeof at least 2 hours; wherein an amount of said fluoride in said calciumoxide composition is reduced compared to an amount of said fluoride insaid limestone.

In an embodiment of the method, the calcining occurs at a temperature of1600° C. or greater.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows certain impurity levels for limestone having variousbrightness levels.

FIG. 2 shows certain impurity data for high quality L70 brightnesslimestone originally and calcined at four different temperatures.

FIG. 3 shows impurity data comparing reductions for lower quality (<L60)limestone with higher quality (>L60) limestone for certain impuritiescalcined at four different temperatures.

FIG. 4 shows impurity data for a number of impurities for a low quality(L52) limestone. Two rows are highlighted illustrating the removal oflead alone and lead in conjunction with commonly controlled leadcompounds (those provided in California Proposition 65).

FIG. 5 shows impurity data for a number of impurities for another lowquality (L55) limestone. Two rows are highlighted illustrating theremoval of lead alone and lead in conjunction with commonly controlledlead compounds (those provided in California Proposition 65).

FIG. 6 shows impurity data for a number of impurities for a high quality(L76) limestone. Two rows are highlighted illustrating the removal oflead alone and lead in conjunction with commonly controlled leadcompounds (those provided in California Proposition 65).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present systems and methods relate to the purification of calciumoxides and any further resultant downstream materials made from thosecalcium oxides such as, but not limited to, calcium hydroxide, which areproduced from limestone including impurities. The term “purify” in thiscontext and application means to increase the degree of purity, which aperson of ordinary skill will understand typically does not necessarilymean that the resulting compound is 100% pure and therefore purificationof material herein does not mean to completely remove an impurity(although it can) but simply to reduce it's total amount by acommercially valuable and/or detectable quantity possibly to a valuewhich is no longer detectable by current instruments used to evaluatematerial.

Further, “limestone” in this disclosure will generally mean a naturallyoccurring form of calcium carbonate (CaCO₃). As such, limestone, as anaturally occurring material, will typically not consist of pure calciumcarbonate, but will comprise additional materials whose removal isdesired and those materials will typically be different depending onspecifics of where and how the limestone was obtained. It should berecognized, however, that “limestone” as that term is used herein canmean a manmade form of calcium carbonate which includes impurities (forwhatever reason) and is desired to be purified. It should be recognizedthat while “calcium carbonate” as used herein will principally be usedto refer to that specific material as a component of limestone, calciumcarbonate may sometimes refer to both the specific compound andlimestone at the same time.

Materials which are to be preferably removed are typically those whichare considered dangerous to humans, which can cause lack of brightness,and/or which can create problematic reactions or byproducts when calciumoxide or downstream materials made from calcium oxide such as, but notlimited to, calcium hydroxide are utilized in known or newly developedindustrial processes. These materials will typically include, but arenot limited to: cadmium, cerium, cesium, cobalt, copper, fluoride, lead,lithium, potassium, rubidium, silver, sodium, thorium, and zinc as wellas chemical compounds of these materials such as, but not limited to,oxides and salts.

It should be understood that in order to perform purification on theoutputs of limestone, it is first desirable to determine a baseline ofhow much impurity is originally present. In particular, limestone withhigher lead content initially will generally show an increased reduction(as a literal amount) compared to a limestone with a reduced amount if aprocess is successful but this is by no means required. In anembodiment, a process will provide an end amount of impurity which isrelatively independent of the amount of the impurity initially presentbut which amount is reduced but this is by no means required and thereduction may be of generally similar percentages regardless of theinitial amount provided.

To determine a likely starting point for how high initial impuritylevels are of certain target impurities that are to be removed,brightness is used as a general indicator of degree of impuritypresence. Lower brightness can be, but is not necessarily, indicativethat a limestone includes an increased number of impurities generally.However, testing of specific limestone will reveal specific impuritieswith concentrations of interest.

A commonly used scale for brightness in the United States is the TAPPIscale, also sometimes known as the GE scale. This standard was initiallydeveloped as a measure of diffuse blue reflectance, primarily for use inpulp bleaching, and was performed by early instruments, such as theGeneral Electric (GE) Photovolt. This early association with GE has ledto the industry shorthand “GE brightness” to refer to this measuringtechnique. GE brightness is generally a measure of the reflectivity ofvisible light (with a wavelength of about 457 nm) from a substance whenobserved at a 45° angle.

The Technical Association of the Pulp and Paper Industry (TAPPI) is nowthe keeper of this standard, and it is sometimes referred to as theTAPPI scale. Various TAPPI standards exist, including T452 and T457.This scale generally uses directional brightness. Because this measuredepends on direction, variance in the results can be induced by themeasurement angle. Other measures of brightness may be used as well, orinstead, of the TAPPI/GE scale. For example, outside of the UnitedStates, diffuse brightness measures such as ISO C Brightness (ISO2970-1, TAPPI T525) or ISO D65 (ISO 2970-2) are generally preferred. GEbrightness is just one measure of products brightness or whiteness, andalternate tests include the “Hunter L” and “Hunter Y” scales among othermeasures. For sake of clarity, the directional brightness technique willbe referred to herein as “GE brightness.”

GE brightness for limestone can range substantially, but is typicallyaround 80 on the GE scale (which measures from 1-100 with 100 beingconsidered pure white) for fairly pure materials. This level of GEbrightness is often interpreted as “light grey” in color. However,so-called “high-brightness” limestone can have a GE brightness oftenexceeding 90, while low brightness materials are often in the 40-60range and can be a dark grey and are believed to include a large numberof impurities.

As low brightness limestone is unusable for many common application ofcalcium carbonate (such as paper and paint production) it is often whatis desired to be used in calcium oxide and calcium hydroxide production.However, often decreases in brightness are indicators that certainundesirable impurities are present in greater numbers as indicated abovewhich can render that limestone unsuitable source material. Thisexpectation is validated as shown in FIG. 1 which illustrates how lowbrightness limestone, using a Hunter-L score, typically have a higherquantity of lead and other undesirable impurities.

However, it should be recognized that brightness level, while a goodproxy for total impurity levels, is not an exact, or sometimes evenaccurate, indicator of any particular impurity level. However, treatmentmethods which can work across a broader spectrum of initial inputs areoften more commercially valuable as they can be used more ubiquitously.Therefore, brightness scores are used herein as an identifier fordifferent types of limestone input having different levels of impuritiesto illustrate examples of how a variety of different limestonecompositions can be purified.

Calcination, or the exposure of a compound to heat to create thermaldecomposition, commonly serves to not only decompose the sourcecompound, but can also be used to remove impurities from limestone. Insome instances, impurities are simply vaporized and separate from thecompound because the temperature is raised above their boiling point. Inother instances, chemical bonds break down and the impurities separateout. Fluoride (F⁻), for example, is known to be removable usingcalcination as is illustrated in, for example, U.S. Pat. Nos. 2,021,663and 2,127,626. However, other impurities have previously not beenresponsive to removal via calcination. These include, but are notlimited to: cadmium, cerium, cesium, cobalt, copper, lead, lithium,potassium, rubidium, silver, sodium, thorium, and zinc. Some of theseimpurities, such as lead, are harmful to humans and are an impuritywhose removal is highly desired particularly if the calcium oxide is toeventually be used in the purification of water or in the food industry.

Calcination is, roughly speaking, a thermal treatment in which acompound is subjected to high temperatures in a typically low-oxygenenvironment, which results in thermal decomposition. Thermaldecomposition is a chemical process in which a compound is “de-composed”from its constituent parts into its components by breaking chemicalbonds. As described above, one of the most common applications ofcalcination is converting calcium carbonate to quicklime through theapplication of heat. Calcination is typically carried out in a furnaceor reactor, sometimes also called a kiln or calciner.

Calcination typically takes place at the temperatures at which thecompound in question breaks down. In chemical terminology, this is thetemperature at which the standard Gibbs free energy for the calcinationreaction is zero. For limestone calcination, this is approximated asΔG°_(r)=177,100−158 T (J/mol), and the standard free energy of thereaction is 0 when the temperature is equal to 1121 K, or about 848° C.(1,558° F.). Thus, it is widely accepted that the ideal maximum firingtemperature for limestone is around 900° C. (1,652° F.). A secondcomponent of calcination is the “soak” time. “Soaking” generally refersto the practice of maintaining the furnace at a maximum firingtemperature for a specified period of time.

The present systems and methods comprise calcining a calcium carbonatesource generally in a kiln at a higher temperature and generally longersoak time than has typically been used. This disclosure primarilyconcerns calcium carbonate in limestone, but may be applicable to anypolymorph or other minerals or stone containing calcium carbonate,including, but not necessarily limited to, chalk, marble, andtravertine.

The source calcium carbonate (typically as a part of a crushed or groundlimestone) is calcined in the kiln, by raising the temperature to apredetermined maximum temperature, and then soaking the compound at thatpredetermined maximum temperature for a predetermined soak time.Whereas, in the prior art, the optimal temperature for calcination canbe calculated to be a maximum of around 900° C., the present disclosurecontemplates much higher temperatures of at least 1,100° C., at least1,200° C., at least 1,300° C., at least 1,400° C., at least 1,500° C.,or at least 1600° C. Without being limited to any one principle ofoperation, it is believed that these highly elevated temperatures causeheavier elements to vaporize, or otherwise break chemical bonds, causingthe impurities to boil or fall out of the compound.

In an embodiment, the temperature is not only increased but the soaktime is also additionally increased. Whereas a soak time of about onehour is standard in the industry for calcium carbonate calcination, inan embodiment, the soak time in the present systems and methods is aboutdoubled to about two hours. In further embodiments, the soak time may bemore than two hours and may be substantially increased depending on thetarget impurity to be removed.

Experimentally, the combination of elevated temperature and increasedsoak time achieves progressively lower amounts of heavy elementinclusion, particularly lead, as temperature is increased. It isbelieved that this technique has similar implications for otherelements, notably the mid-to-high period alkali metals, including, butnot necessarily limited to, cesium, potassium, and rubidium. Some gainswere also observed with silver and other potentially problematicimpurities.

Multiple experiments were conducted using samples of limestone having abrightness of L70, a higher grade stone with fewer impurities of thetarget elements as illustrated in FIG. 1 . The experiments wereconducted at a longer soak time (about 2 hours) in all cases but atvarying temperatures to assess the degree of purification achieved withvarying temperature ranges. The experiment results are summarized inFIG. 2 .

As shown in FIG. 2 , at a calcine temperature of 1,100° C., the initialL70 sample yielded a product containing 0.41 ppm lead. At a calcinetemperature of 1,200° C., the initial L70 sample yielded a productcontaining 0.31 ppm lead. At a calcine temperature of 1,300° C., theinitial L70 sample yielded a product containing 0.19 ppm lead. At acalcine temperature of 1,400° C., the initial L70 sample yielded aproduct containing 0.13 ppm lead. However, the opposite effect wasobserved on certain other elements. For example, increases in content ofcertain other elements after calcination at the above temperatures wasseen, notably in the first transition metals series 4 metals (e.g.,iron, nickel, and titanium) although those results are not shown in FIG.2 . For this reason, the proposed methods and systems may not besuitable for removal of first transition metals while it may be suitablefor removal of heavier metals and particularly mid-to-high period alkalimetals.

It should be apparent that the ability to remove such impurities throughcalcination can be very valuable where calcination is to be performedanyway. For the generation of calcium oxide (or calcium hydroxide) fromsource calcium carbonate, calcination is a necessary step and thecalcination contemplated herein utilizing higher temperatures and/orlonger soak times allows for impurity removal without the need to exposethe various materials to additional compounds or to additional processeswhich can often be both commercially undesirable and damaging toresultant compounds.

The system and methods contemplated herein can be performed usingexisting systems dramatically simplifying their use. Further, calciningcan be performed in the higher temperature ranges and longer soak times.Calcining need not be performed by first raising the temperature to alower or more traditional value (e.g. around 900° C.) and then laterraising the temperature to that listed although that may be performed inalternative embodiments. Calcining, as shown in the discussedexperiments, may be performed at the specifically listed highertemperature (e.g. above 1100° C.) simplifying the calcining process.

To determine if die impurity removal continues across a variety ofdifferent source calcium carbonate, calcination at the same temperatureranges and times as for the sample of FIG. 2 were performed on twosamples of varying brightness levels (<1.60 and >L60). It was expectedthat darker samples would have a higher level of inclusions,particularly of dark metals such as lead as indicated in FIG. 1 . Thepresent methods and systems clearly provide for increased removal of thelisted target impurities, but it can also be noted that similar absolutelevels of lead were also obtained for all three samples (those of FIGS.2 and 3 ). This implies that the systems and methods contemplated hereincan produce similar resultant materials regardless of input allowing thesystems and methods to produce a relatively uniformly pure outputmaterial (with regards at least to lead) regardless of the impuritycontent of the initial limestone input. This is, however, not required.

Additional experiments were carried out and the results are shown inFIGS. 4, 5 , and 6. In these tables the amounts of a number of compoundsare provided but the focus (shown in the highlighted rows) was on leadspecifically and the combination of lead and other lead compounds whichare commonly thought to be harmful to humans. In the tables of FIGS. 4-6, the lead and lead compounds are those included in California'sproposition 65 warnings as lead and lead compounds. FIGS. 4-6 alsoincluded further increased temperature ranges to determine if highertemperatures resulted in additional decreases.

As can be seen in FIGS. 4-6 , lead and lead compounds were generallyreduced in an increasing amount as temperature generally increased.Further, this reduction occurred in all three grades of sourcelimestone. In many cases, the reductions were dramatic with lead notbeing detected (“ND”) in the highest temperature calcination 0600° C.)in all three cases. Thus, reductions in lead to 75% or less, 50% orless, 25% or less, or to undetectable amounts are all possible. It,therefore, appears clear that high temperature calcination can providefor an increased purity of calcium oxide at least with regards to leadand lead compounds and can result in a generally complete removal oflead within the detectability of present instruments. It is expectedthat increased soak time could provide further increases, but longersoak times may not always be as commercially feasible as increasedtemperature.

FIGS. 4-6 also show a number of other important reductions. Cesium inparticular also shows a near general increase in reduction astemperature generally increased. Cadmium, Potassium, and Rubidium alsoshowed general reductions particularly at the highest temperatureranges. Further, at the end of each chart, reductions in fluoride arealso noted. Fluoride reductions also showed a general increase inreduction as temperature increased indicating that the process of veryhigh temperature calcination can server to reduce fluoride without theneed to add additional compounds or calcine at a lower temperature andthen perform impurity removal at a higher one.

Based on the above, it would appear clear that increased calciningtemperature is an effective way to remove a number of impurities fromcalcium oxide and other products produced from limestone. The processcan also be enhanced through increased soak time of 2 hours or more, 3hours or more or 4 hours or more. Alternatively, increased soak time maybe substitutable for additional increases in temperature. The processesappear particularly valuable for the removal of lead and lead compoundsallowing even low quality natural limestone (as measured by brightnessscore) to be formed into calcium oxide or calcium hydroxide that may besuitable for industries where such materials were previously unsuitable.

While the above systems and methods are preferably used in theproduction of quicklime (calcium oxide) with reduced impurity load,particularly with regards to lead, this process can also be used toprovide source quicklime for the production of lime hydrates withreduced impurity load which can make potentially lower brightness sourcelimestone suitable as a feedstock for final product applications ofcalcium hydrates, such as food and water purification, where impuritiessuch as lead may have previously rendered such feedstock unsuitable.

While the invention has been disclosed in conjunction with a descriptionof certain embodiments, including those that are currently believed tobe the preferred embodiments, the detailed description is intended to beillustrative and should not be understood to limit the scope of thepresent disclosure. As would be understood by one of ordinary skill inthe art, embodiments other than those described in detail herein areencompassed by the present invention. Modifications and variations ofthe described embodiments may be made without departing from the spiritand scope of the invention.

It will further be understood that any of the ranges, values,properties, or characteristics given for any single component of thepresent disclosure can be used interchangeably with any ranges, values,properties, or characteristics given for any of the other components ofthe disclosure, where compatible, to form an embodiment having definedvalues for each of the components, as given herein throughout. Further,ranges provided for a genus or a category can also be applied to specieswithin the genus or members of the category unless otherwise noted.

Finally, the qualifier “generally,” and similar qualifiers as used inthe present case, would be understood by one of ordinary skill in theart to accommodate recognizable attempts to conform a device to thequalified term, which may nevertheless fall short of doing so. This isbecause terms such as “rectangular” are purely geometric constructs andno real-world component is a true “rectangular” in the geometric sense.Variations from geometric and mathematical descriptions are unavoidabledue to, among other things, manufacturing tolerances resulting in shapevariations, defects and imperfections, non-uniform thermal expansion,and natural wear. Moreover, there exists for every object a level ofmagnification at which geometric and mathematical descriptors fail dueto the nature of matter. One of ordinary skill would thus understand theterm “generally” and relationships contemplated herein regardless of theinclusion of such qualifiers to include a range of variations from theliteral geometric or other meaning of the term in view of these andother considerations.

The invention claimed is:
 1. A method of producing calcium hydroxide,the method comprising: providing limestone including calcium carbonateand impurities, said impurities including at least one of: cadmium,cerium, cesium, cobalt, copper, fluoride, lead, lithium, potassium,rubidium, sodium, thorium, or zinc; and calcining said limestone toproduce a calcium oxide composition, said calcining occurring at atemperature of 1200° C. or higher and having a soak time greater than 1hour; slaking said calcium oxide composition to produce calciumhydroxide; wherein an amount of said impurities in said calcium oxide isreduced compared to an amount of said impurities in said providedlimestone.
 2. The method of claim 1, wherein said calcining occurs at atemperature of 1300° C. or greater.
 3. The method of claim 1, whereinsaid calcining occurs at a temperature of 1400° C. or greater.
 4. Themethod of claim 1, wherein said calcining occurs at a temperature of1500° C. or greater.
 5. The method of claim 1, wherein said calciningoccurs at a temperature of 1600° C. or greater.
 6. The method of claim 1wherein said soak time is 2 hours or greater.
 7. The method of claim 1wherein said amount of said impurities in said calcium oxide is no morethan 75% said amount of said impurities in said provided limestone. 8.The method of claim 7 wherein each impurity in said impurities in saidcalcium oxide is no more than 75% of said impurity in said providedlimestone.
 9. The method of claim 1 wherein said amount of saidimpurities in said calcium oxide is no more than 55% said amount of saidimpurities in said provided limestone.
 10. The method of claim 9 whereineach impurity in said impurities in said calcium oxide is no more than55% of said impurity in said provided limestone.
 11. The method of claim1 wherein said amount of said impurities in said calcium oxide is nomore than 25% said amount of said impurities in said provided limestone.12. The method of claim 11 wherein each impurity in said impurities insaid oxide is no more than 25% of said impurity in said providedlimestone.
 13. The method of claim 1 wherein none of said impurities insaid calcium oxide is detectable.
 14. A method of producing calciumhydroxide, the method comprising: providing limestone including calciumcarbonate and impurities, said impurities including cadmium, cerium,cesium, cobalt, copper, fluoride, lead, lithium, potassium, rubidium,sodium, thorium, and zinc; and calcining said limestone to produce acalcium oxide composition, said calcining occurring at a temperature of1100° C. or higher and having a soak time greater than 2 hours; slakingsaid calcium oxide composition to produce calcium hydroxide; wherein anamount of said impurities in said calcium oxide is reduced compared toan amount of said impurities in said provided limestone.
 15. The methodof claim 14, wherein said calcining occurs at a temperature of 1200° C.or greater.
 16. The method of claim 14, wherein said calcining occurs ata temperature of 1300° C. or greater.
 17. The method of claim 14,wherein said calcining occurs at a temperature of 1400° C. or greater.18. The method of claim 14, wherein said calcining occurs at atemperature of 1500° C. or greater.
 19. The method of claim 14, whereinsaid calcining occurs at a temperature of 1600° C. or greater.
 20. Themethod of claim 14 wherein said amount of said impurities in saidcalcium oxide is no more than 75% said amount of said impurities in saidprovided limestone.